In the past few decades, hundreds of millions of people have placed contactless cards in their wallets, purses or handbags. The first contactless cards were low-cost, credit card-sized plastic cards that were used to control people's access to trains, trams or buses. It is fast and convenient to use, so it is of course popular with consumers. In the past decade, contactless technology has been extended to payment cards (such as debit cards, credit cards, and prepaid cards) to speed up transactions in fast-paced retail environments such as newsstands, coffee shops, and convenience stores. In fact, the conditions for the widespread adoption of non-contact technology have been improved: * Consumers understand and accept this technology * Non-contact card readers have been installed in large quantities (at point-of-sale terminals, ticket offices and other locations) * Thousands of retailers, banks, transportation network operators and other organizations have used this type of contactless communication infrastructure for payment management and access control. However, one important element has been overlooked: many consumers carry a smartphone with them wherever they go. In theory, it can simulate contactless cards, allowing users to replace multiple cards with one device. For merchants and transport providers, this is highly desirable: replacing the card with a downloadable application can save the cost of providing the card; in addition, when the consumer feels more convenient, it will accelerate the popularity of the technology. This will help merchants and transportation service providers to increase revenue and improve operational efficiency. Mobile phone manufacturers are also eager to implement contactless technology in their devices to increase the value of their devices and increase their appeal to consumers. The first attempt at this technology was in the early 21st century, due to the release of Near Field Communications (NFC). NFC technology is a standard technology supported by the NFC Forum. It can support the reader mode just like the emulation card, enabling the device to read NFC tags and peer-to-peer mode. But early applications did not properly consider the electrical and mechanical limitations of the phone. In particular, as an NFC card reader, compared to a dedicated NFC card reader, such as a contactless payment terminal, the mobile phone provides much less power and space available for the antenna. It is quite difficult to make the phone achieve comparable performance to the reader. However, what's more important is the lack of a mobile app that is popular with customers. Simply put, mobile phone users don't know what to do with the NFC reader in their pocket. If the phone can be used as an NFC tag, you can simulate a variety of contactless cards. At the same time, it will become a popular and used app: consumers can not only use their favorite equipment to enter and exit buildings, trains and buses, but also use it to pay for stores, use coupons and accumulate points. So, how do smartphone manufacturers implement reliable NFC tagging? Let's first understand how NFC tags communicate with readers. Passive load modulation for contactless cards NFC technology requires inductive coupling of a pair of antennas. The coupling coefficient is k, representing the degree of susceptibility of a pair of card reader/card antenna groups. The value of the coefficient is between 0 and 1, depending on the geometry of the antenna and the distance between the antennas. Under normal circumstances, when the antenna of the card is smaller than the reader antenna, the coupling coefficient is proportional to the surface area of ​​the card antenna regardless of the specific distance from the reader antenna: the larger the surface, the larger the coupling coefficient. When using the general "Passive Load Modulation (PLM)" method to transfer data, the value of k usually needs to be between 0.03 and 0.3. A contactless NFC transaction is a series of reader commands, each of which has a response from the card. PLM is used for contactless cards and all existing NFC phones except the Apple iPhone 6 and iPhone 6 Plus, which operate by switching to the card antenna's passive (resistive or capacitive) load (see Figure 1). A default load responds to the non-modulated state, and the switching load responds to the modulation state. When the card reader and card antenna are inductively coupled, the reader of the card reader senses these load changes and decodes them to extract information from the signal. Figure 1: Circuitry for passive load modulation. When making an NFC system, it is important to take into account the magnitude of the load modulation: this is the difference between the modulated and unmodulated voltages sensed by the reader receiver (see Figure 3). If this amplitude falls below a certain minimum, the receiver will not be able to reliably sense the card signal modulation. All other parts are equal, and the larger the value of k, the greater the load modulation. When used in a contactless card, the PLM can stably output a sufficient load modulation amplitude. The standard size of a contactless card is generally ID-1, which is a credit card size. A large antenna is embedded in it, which provides a 6 cm receiving range for today's card readers. Indeed, the performance of the contactless card system is excellent, and users can implement ticket checking in the public transportation system without having to take the contactless card out of the wallet or handbag. In fact, consumers expect non-contact systems to operate more quickly, in real time, and completely reliably, allowing their contactless devices to access the reader from all directions, whether by hand or when the device is completely hidden. Wallet or purse. This poses a great challenge for smartphone makers. Because the phone contains many radios and antennas, and is covered in metal. The phone's circuit board is covered with various components, and the larger display screens and batteries that consumers demand are taking up more and more space. Such an environment is completely unsuitable for PLM. The cramped space size can only accommodate tiny antennas. A wide variety of metals and interfering RF signals can also severely affect the ability of a cell phone and reader antenna to couple. The result of this situation is a bad consumer experience: transactions often fail or take a few seconds to complete, and consumers are forced to take their phones out of their bags and carefully approach the card reader, which makes them feel Very inconvenient. How to compensate for low coupling coefficient Design constraints have hampered handset manufacturers' attempts to increase the value of k. As indicated above, these attempts have required a significant increase in handset NFC antenna area. Therefore, in order to improve the performance of the card emulation mode of the mobile phone, the related efforts are mostly focused on increasing the load modulation range. Now, we have found a feasible method called Active Load Modulation (ALM). The ALM uses the battery power of the mobile device to power it. In ALM, a carrier signal synchronized with the magnetic field of the reader is transmitted during the modulation state and is turned off during the non-modulated state (see Figure 2). This mode of operation is called the AND mode. This is referred to as the XOR mode in a more efficient version of the technology in which a signal synchronized with the magnetic field of the reader is transmitted during the modulation state and a 180° phase is transmitted during the non-modulation state. Shift the signal. Compared to the AND mode, the XOR mode doubles the load modulation strength sensed by the reader. In this way, a much smaller antenna can be used. The ALM signal is coupled to the antenna of the reader. The ALM signal is coupled to either increase or decrease the signal of the reader to form a load modulated signal based on its phase difference with the magnetic field of the reader. The main advantage of ALM is that it achieves the same load modulation amplitude as PLM devices with a coupling factor of less than 100 times (see Figure 3). Figure 2: Active load modulation circuit diagram. Figure 3: ALM can use an antenna that is 100 times larger than a typical PLM system antenna. A typical non-contact card antenna has an area of ​​4,000 mm2 and can successfully implement PLM. The ALM circuit can achieve the same consumer experience with the same load modulation amplitude at the reader receiver with just 40mm2 of antenna. For handset manufacturers, the antenna cost of this size is close to zero – it can even be used to print printed circuit board antennas on the motherboard. It does not require ferrite shielding. In comparison, the cost of a large antenna with ferrite and connectors is nearly $1. Furthermore, the NFC antenna can be placed in the best position for the user. For example, when placed near the rear lens, the user only needs to touch the top of the phone to the card reader. Relatively speaking, large antennas placed in the back cover or battery of a smartphone are the cause of user disappointment. In general, they will hold the phone in their hands, so the antenna of the phone cannot be accurately aligned with the reader antenna. The transaction fails when they sweep the phone over the front of the contactless reader. So users must learn to move the phone very close to the card reader, and contactless transmission can be successful—this action is quite different from the experience of using a contactless card. Figure 4: A small antenna can be easily placed in the device to provide the best user experience. In Figure 4, the reader antenna is marked in yellow. The large antennas required for PLM-based solutions are marked in orange and are the place to hold the device. In contrast, the small antenna used in the ALM solution is marked in red, and its location is ideal for successful NFC transactions. ALM is also well suited for use in wearable devices due to the ability to use very small antennas. Wearable devices have limited space and can only accommodate small antennas, but PLM solutions with small antennas do not provide a satisfactory range of communication. Improvement of interoperability between traffic and payment system readers A series of NFC tag analog ICs developed by Ams have adopted the principle of ALM operation, and all products use its "enhanced NFC" technology. When applied to the latest smartphones, the enhanced NFC uses a 40mm2 antenna to match the performance of the contactless card. However, the reduction in antenna size is not the only benefit of enhanced NFC. It also supports various electrical parameters of load modulated signals, including: * Automatic Power Control function, when the phone is close to the card reader (in other words, when the coupling coefficient becomes high), this function can reduce the output voltage and prevent the reader receiver from saturating. * Configure the phase difference of the Active Load Modulation signal according to the magnetic field of the reader, which can improve the reliability of the transaction when the mobile phone leaves the range of the reader. * Sensitivity that can be set and antennas that support a variety of different sizes. The extremely low sensitivity ensures that the reader-to-card link never limits the trading range. * Accurate timing mechanism, which ensures that the delay between the reader command and the card response meets the stringent limits imposed by the contactless standard. This mechanism compensates for various changes that occur at the NFC controller side. * Automatic Gain Control, which provides correct demodulation of the reader signal and accommodates the design of the terminal, especially for public transportation systems. All of these features are dynamically configured to provide higher interoperability throughput rates than existing NFC solutions. When applied to payment procedures, a 100% pass rate can be achieved. In addition, Enhanced NFC technology provides a way for foundry manufacturers to adapt to the inconsistency of installed readers due to the constant modification of the contactless payment card transaction standard EMVCo. (This is not unusual for point-of-sale terminals that have been in operation for a decade.) As aging, the performance of readers will change, and differences in reader performance may also result from inconsistencies in manufacturing and installation. Sex. A very small antenna can also avoid the problem of misalignment of the reader antenna, which can occur with large antennas. EMVCo's requirements for contactless communication range have been relaxed from 4cm to a minimum of 2cm, mainly for NFC handsets with poor performance PLM solutions. And this is almost impossible to meet the expectations of users. Fortunately, this relaxed standard is not necessary in the future, because enhanced NFC will make NFC phones as good or better as traditional contactless cards. Ams' NFC booster IC-AS3922 provides ALM technology for UICC and microSD NFC connection cards. This allows non-contact cards to be emulated in phones and small accessories that do not have NFC functionality. The AS3923 and AS39230 are integrated into the electronics as an add-on to the NFC controller and are primarily used to replace the analog front end of the controller (see Figure 4). The AS39230 also supports NFC's active point-to-point mode and card emulation. In the actual operation of ams, ALM has little effect on the battery operating life, because the enhanced NFC component will maintain the power saving mode until it detects the occurrence of contactless transactions. Digital IC manufacturers are also trying to implement ALM. However, the analog circuits required for ALM systems are very unsuitable for the ultra-small circuit characteristics of advanced digital ICs. Therefore, the load modulation performance of ALM's digital security components (Secure Elements) or NFC controllers is integrated. Very poor. In contrast, the pure analog circuitry used by ams electronic components provides the best performance to meet the expectations of consumers who are accustomed to using convenient contactless cards for contactless technology, so this component has been used in today's market. The top smartphones. Figure 5: Enhanced NFC is used in the add-on chip (AS3923) of the main NFC controller. in conclusion Contactless cards are now widely used in public transportation systems, as well as in contactless payment transactions in stores. Hundreds of millions of consumers carry a smartphone with them, in theory it can emulate a smart card, allowing users to replace multiple cards with a single device that they carry with them. To achieve contactless card functionality in a smartphone environment is arguably full of challenges. Non-contact cards Passive load modulation methods for communicating with contactless readers require the use of large antennas and provide a good environment for RF signals. However, smartphones have a small space and a crowded structure, which is covered with metal objects and subject to radio frequency interference. As a result, smartphone and wearable device manufacturers have turned to another way to implement NFC contactless communication: active load modulation. This article is intended to describe how active load modulation works and to illustrate the benefits of this approach for use in smartphones.
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